project title: Spin Current Turbulence
project leader: Prof. H. Godfrin
access given (in days): 120
access used (in days): 113
local host: Prof. Grisha Volovik
home institution: CRTBT/CNRS
country of institution: France
starting date (yyyy-mm-dd): 2004-08-01
The NMR properties of superfluid 3He-B led to the observation of a new quantum phenomenon. The inhomogeneity of the precession frequency creates spin supercurrents which redistribute the magnetization in such a way that magnetization is precessing uniformly. The main property of these precessing states is the quantum coherence of the phase of the Larmor precession throughout the sample, which persists even in a rather inhomogeneous external magnetic fields. This phenomenon was the first realization of spin superfluidity. Later experiments demonstrated that the spin-superfluid state of 3He-B looses its stability below about 0.4 Tc. The problem was to find the proper reason of the instability and identify the state below 0.4 Tc.
In this project it was previously confirmed that the spin-superfluid precessing state of 3He-B, with its off-diagonal long-range order and quantum coherence, represents the Bose-Einstein condensation (BEC) of spin waves -- magnons. This is actually the first realization of the BEC of excitations in magnets. Using this picture of magnon condensation, the new phenomena have been found and identified in 3He liquids. In particular, theoretical prediction of Coleman of Q-balls in relativistic quantum fields has been realized experimentally in 3He-B, see Yu.M. Bunkov, G.E. Volovik, "Magnon condensation into Q-ball in 3He-B", Phys. Rev. Lett. 98, 265302 (2007). The observed condensed-matter analogs of relativistic Q-balls are responsible for an extremely long lived signal of magnetic induction -- the so-called Persistent Signal -- observed in NMR at the lowest temperatures, where the bulk BEC of magnons is unstable due to catastrophic relaxation. Thus the Q-ball is another representative of magnon condensate in 3He-B characterized by a phase coherent precession of nuclear spins. Durin the reporting period, in the paper by J. Elbs, Yu. M. Bunkov, E. Collin, H. Godfrin, and G.E. Volovik "Strong orientational effect of stretched aerogel on the 3He order parameter", arXiv:0707.3544, it was found that deformation of aerogel strongly modifies the orientation of the order parameter of superfluid 3He confined in aerogel. We used a radial squeezing of aerogel to keep the orbital angular momentum of the 3He Cooper pairs in the plane perpendicular to the magnetic field and observed a clear experimental evidence of the destruction of the long-range order by random anisotropy - the Larkin-Imry-Ma effect -- in 3He-A. In 3He-B we observed and identified new modes of NMR, which are impossible to obtain in bulk 3He-B. One of these modes is characterized by a repulsive interaction between magnons, which is suitable for the magnon Bose-Einstein condensation. This mode represents another type of BEC of magnons in 3He-B. Very recently the observation of BEC of magnons has been preliminary reported for 3He-A in the squeezed aerogel (see review paper by Yu.M. Bunkov and G.E. Volovik, "Bose-Einstein condensation of magnons in superfluid He", J. Low Temp. Phys. 150, 135--144 (2008)).